The present invention relates to fluid forming. In the present specification, the term fluid forming relates to the general process of deforming a material, usually in the form of a tubular blank, by the application of fluid pressure. The fluid may be a liquid, a gas or a fluidized solid, e.g. solid particles which collectively act as a fluid.
Hydroforming is a commonly used fluid forming process using water as a pressurized fluid. It is a well known prior art metal working process that uses a pressurized fluid to expand a closed channel work piece outwardly into conformance with a die cavity. For example, a metal tube may be hydroformed to a desired tubular shape. The metal tube is placed between a pair of dies having cavities which define the desired resultant shape of the tube. The ends of the tube are accessible through the die and a seal is connected to the ends of the tube so that pressurized fluid injected into the tube forces the tube to expand and conform to the shape defined by the die cavity.
The result of the hydroforming process is essentially to deform or “blow up” the tube so that it conforms to the inner surface of the mold or tool. Some limitations on the use of hydroforming are present, since the material forming the tube can be stretched or deformed only in accordance with respective material limits. Furthermore, assuming that a tubular member to be hydroformed has an approximately constant thickness, it will be stretched or deformed evenly over its entire surface area. Thus, hydroformed elements have certain advantageous strength characteristics.
Polymers, by definition, are long chain molecules in which the atoms are bound to one another by means of strong covalent bonds. Hence one would expect exceptionally high strength and stiffness values in the chain direction since the applied load would then be opposed by the covalent bonds themselves. On the contrary, most of the commercial polymers exhibit strength and stiffness values far below their theoretical limits. It is established that the modulus values of most of the commercial polymers are at least an order of magnitude less than their theoretical limits, thus severely limiting their use in many structural or load bearing applications. One of the many ways to improve engineering properties lies in the preparation of oriented polymers.
The need for oriented polymers has led to the development of several orientation techniques. Conventionally, three types of molecular orientation can be introduced into any isotropic polymeric system. One type of molecular orientation is a uniaxial orientation with a preferential alignment of the polymer molecules along the direction of application of deformation force. Typical processes which lead to uniaxial orientation are cold drawing and cold extrusion. Another type of molecular orientation is a biaxial orientation. In this case, the polymer molecules are preferentially aligned along two different deformation directions usually perpendicular to one another. Film blowing processes have been normally used to create biaxial orientation. A third type of molecular orientation is triaxial orientation. When a uniaxially oriented polymer is rolled under suitable conditions, one of the crystallographic planes usually lies parallel to the molecular axis and becomes oriented within the plane of rolling, yielding a triaxially oriented polymer. U.S. Pat. No. 5,411,805 to Magill, for example, discloses triaxially oriented polymer membranes or thin films produced by a rolltrusion process.
Biaxially oriented polyolefin films, such as, for example, biaxially oriented polypropylene films (BOPP films), are widely used as films for packaging, since they are excellent in moisture properties, strength, clarity, and surface gloss. They are generally made by a method comprising solid state orientation. The majority of commercially available biaxially oriented polypropylene films are produced by the flat film or tenter stretching process. Stretching to orient a thermoplastic material is widely utilized within the art since it is well known that an oriented material exhibits increased tear resistance in the direction transverse to the direction of stretching and orientation.
The use of oriented films is widespread, particularly with films comprised of semi-crystalline thermoplastic polymers. These oriented films are characterized by high tensile strength and low to moderate elongation. Orientation can also influence crystalline order and hence the melting or softening point of an oriented polymer. Examples of oriented films are disclosed in U.S. Pat. No. 6,514,597 to Strobel et al. disclosing embossed oriented thermoplastic films and U.S. Pat. No. 6,132,668 to Baars et al. disclosing the formation of thick films having a biaxial molecular orientation.
U.S. Pat. No. 6,631,630 issued to Pourboghrat et al. discloses an apparatus and a method for hydroforming materials such as sheet metal or composite sheets of thermosetting or thermoplastic polymers. In particular, Pourboghrat et al. disclose an apparatus and a method for shaping complex structures using composite sheets such as continuous-fiber or woven fiber composites with limited wrinkling or rupture of the composite sheets during the shaping process. The hydroforming process and apparatus disclosed by Pourboghrat et al. describes the hydroforming of sheet structures over a punch die using a pressurized fluid as a counter “die” to form a hydroforming cavity. As the die punch travels into the sheet blank, the blank begins to deform into a hemispherical shape initially and finally into a fully formed part. Nonetheless, Pourboghrat et al. do not describe a hydroforming process of more complex structures such as tubular structures.
U.S. Pat. No. 5,169,587 to Courval, U.S. Pat. No. 5,169,589 to Francoeur et al., and U.S. Pat. No. 5,204,045 to Courval et al. disclose processes for ram extruding unmelted thermoplastic polymers into highly oriented rods of enormous strength. The processes generally require two steps. First, the thermoplastic polymer is extruded conventionally into semi-crystalline polymer billets which can have a variety of profiles, such as solid, round, or rectangular profiles. The billets are then heated to slightly below the melting point and are ram extruded with a very high draw ratio into thin, oriented polymer profiles. A haul off stress between at least 0.5 Mpa and a maximum amount without plastic deformation of the extrudate keeps the profiles from relaxing before they cool.
However, it is desirable to produce stronger polymers that have more complex structures. Thus, it is desirable to produce biaxially and/or triaxially oriented thermoplastic materials by means of a fluid forming process. Fluid forming can yield more complex structures, such as strong tubular structures for use in automotive vehicle doors and frames or energy-absorbing structures.
Hydroforming for shaping metals has been shown to have important advantages over other metal shaping processes, such as reduced tooling cost, increased drawability, and formation of components with greater dimensional stability. It is desirable to produce thermoplastic materials with similar advantages using a fluid forming process.